One generally and widely known example of the process for producing a polyamide through polycondensation of a dicarboxylic acid component and a diamine component includes producing an aqueous solution of a nylon salt from a dicarboxylic acid component and a diamine component and subsequently melt-polymerizing the nylon salt under pressurized conditions.
Another disclosed example of the process includes directly melt-polymerizing a dicarboxylic acid component and a diamine component in the absence of solvent without preparing an aqueous solution of a nylon salt (see Patent Document 1). In this process, the diamine component is added while the temperature of the formed polymer is elevated at the melting point of the polymer or higher. Thus, the process is economically advantageous, since there is no need for removal of water (water of aqueous solution of a nylon salt) or solvent through distillation.
In this production process, it is advantageous from the standpoint of production that the boiling point of the diamine component be equal to or higher than that the melting point of the formed polyamide. When the boiling point of the diamine component is lower than the melting point of the polyamide, polycondensation does not efficiently proceed, due to vaporization of the added diamine. The boiling point of xylylenediamine is about 274° C., which is relatively higher than boiling points of other diamines conventionally used for production of polyamides. For example, hexamethylenediamine has a boiling point of 199 to 205° C. Therefore, the production process including directly melt-polymerizing a dicarboxylic acid component and a diamine component in the absence of solvent is advantageous when xylylenediamine is used.
On the other hand, in the production process including directly melt-polymerizing a dicarboxylic acid component and a diamine component in the absence of solvent, the dicarboxylic acid component and the diamine component readily scatter as monomer components in the vapor phase, resulting in formation of a nylon salt and oligomers in the vapor phase section and other sections of the reactor. The thus-formed nylon salt and oligomers are generally dissolved in water which is generated through polycondensation between the dicarboxylic acid component and the diamine component, whereby the salt and oligomers return to the reaction mixture. Thus, no serious problem occurs in terms of product quality and the production process.
However, when the p-xylylenediamine content of the diamine component is 50 mol % or more, deposits are gradually observed in a pipe that connects a reaction tank with a partial condenser and in the vapor phase section of a reaction pot. As the p-xylylenediamine content increases, the water solubility of the salt formed from the diamine component and the dicarboxylic acid component decreases. This phenomenon is more significant when the p-xylylenediamine content of the diamine component is 70 mol % or more. Since the salt formed from the diamine monomer and the dicarboxylic acid monomer has poor solubility in water formed through polycondensation, the salt cannot be washed out but is deposited in the vapor section of a reaction tank and in a vapor pipe.
Such deposition tends to occur particularly in the vapor phase section of the reaction tank, which hereinafter may be referred to as “the reaction tank vapor phase section,” and in a pipe that connects a reaction tank with a partial condenser, for the following reasons. That is, these members are lower in temperature than other members, and the deposited matter has poor solubility in water at high temperature, resulting in further deposition on the deposits, to thereby form considerable mass of deposits. In addition, when the deposits are heated, the degree of polymerization may increase, or gelation may occur.
When assuming a block form, the deposits may be peeled off from the aforementioned members during production of polyamide and incorporated into the product as solid matter, to thereby deteriorate the quality of the product. Such solid matter causes problems such as clogging of die holes during discharge of the polyamide from the reaction apparatus and interruption of strand formation, whereby stable pelletizing operation is disturbed. Therefore, the reaction apparatus must be periodically opened, and the solid matter must be manually removed or washed off with solvent or the like.
Meanwhile, there is disclosed a method for synthesizing polyamide, which method attains an enhanced productivity by suppressing formation of block-form deposits (see Patent Document 2). In the disclosed method, deposition of solid matter in a stirring apparatus is suppressed by a characteristic design of stirring blades. However, this patent document does not disclose the effects of suppressing deposition of a nylon salt and oligomers in the reaction tank vapor phase section and in a pipe that connects a reaction tank with a partial condenser.
Another document discloses that, in a method for producing a polyamide including a diamine component having a xylylenediamine content of 70 mol % or more, the xylylenediamine containing 20 mol % or more p-xylylenediamine, and 70 mol % or more of adipic acid, deposition of polymer in the reaction tank is suppressed through direct dropwise addition of the diamine component to the dicarboxylic acid component at an elevated pressure range of 0.1 to 0.4 MPaG (see Patent Document 3). Patent Document 3 discloses the effect of suppressing deposition when the polyamide has the aforementioned composition under the above pressure conditions. However, a polyamide resin which may form a salt having poor water solubility; e.g., a polyamide resin including a diamine component having a p-xylylenediamine content of 70 mol % or more, and a dicarboxylic acid component, readily provide deposits. In this case, a satisfactory effect of suppressing deposition cannot be attained even when the polyamide is synthesized under pressurized conditions.
Under the aforementioned circumstances, when a polyamide is produced by directly melt-polymerizing a dicarboxylic acid component and a diamine component in the absence of solvent by using a diamine component having a p-xylylenediamine content of 70 mol % or more, there is demand for a process that can enhance productivity by suppressing deposition of solid matter in the reaction tank vapor section and in a vapor pipe and which can suppress deposition of solid matter in the reaction tank vapor phase section and in a pipe that connects the reaction tank with a partial condenser, in order to reduce incorporation, into the product, of unmelted solid matter originating from the deposits.